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Abstract

In this article, we report the synthesis of single-crystalline nickel silicide nanowires
(NWs) via chemical vapor deposition method using NiCl2·6H2O as a single-source precursor. Various morphologies of δ-Ni2Si NWs were successfully acquired by controlling the growth conditions. The growth
mechanism of the δ-Ni2Si NWs was thoroughly discussed and identified with microscopy studies. Field emission
measurements show a low turn-on field (4.12 V/μm), and magnetic property measurements
show a classic ferromagnetic characteristic, which demonstrates promising potential
applications for field emitters, magnetic storage, and biological cell separation.

Keywords:

CVD; Ni2Si nanowires; Field emission; Ferromagnetic characteristic

Background

With the miniaturization of electronic devices, one-dimensional (1-D) nanostructures
have attracted much attention due to their distinct physical properties compared with
thin film and bulk materials. One-dimensional materials, such as nanorods, nanotubes,
nanowires (NWs), and nanobelts, are promising to be utilized in spintronics, thermoelectric
and electronic devices, etc. [1-5]. Metal silicides have been widely synthesized and utilized in the contemporary metal-oxide-semiconductor
field-effect transistor as source/drain contact materials, interconnection [6], and Schottky barrier contacts. One-dimensional metal silicides have shown excellent
field emission [7,8] and magnetic properties [9-11]. Hence, recently, the synthesis and study of 1-D metal silicide nanostructures and
silicide/silicon or silicide/siliconoxide nanoheterostructures have been extensively
investigated [9,12-18]. Among various silicides, Ni silicide NWs with low resistivity, low contact resistance,
and excellent field emission properties [19,20] are considered as a promising material in the critical utilization for the future
nanotechnology. Thus, plenty of methods have been reported to synthesize Ni silicide
NWs. Wu et al. have formed NiSi NWs by the chemical reaction between coated Ni metal
layers and pre-fabricated Si NWs [13]. In addition, metal-induced growth, chemical vapor deposition (CVD), and chemical
vapor transport method have been successfully applied to synthesize NiSi [21,22], Ni31Si12[20], Ni3Si [23], and Ni2Si [24] NWs, and their physical properties have been investigated. For simplification of
the whole processing, metal chloride compounds such as Fe(SiCl3)2(CO)4[9], CoCl2[11,25], or NiCl2[19] are commonly used as single-source precursors (SSPs) in synthesizing metal-silicide
NWs. In this work, δ-Ni2Si NWs were synthesized via CVD method with SSP of NiCl2. The morphology and yield of δ-Ni2Si NWs can be mastered through parameter control. The δ-Ni2Si NWs were structurally characterized via high-resolution transmission electronic
microscopy (HRTEM). The growth mechanisms of δ-Ni2Si NWs and NiSi phases were identified through structural analysis by X-ray diffraction
(XRD) and TEM. Electrical measurements showed an outstanding field emission property,
and magnetic property measurements demonstrated a classic ferromagnetic behavior of
the δ-Ni2Si NWs.

Methods

The synthesis of the silicide NWs was carried out in the three-zone furnace via a
chemical vapor deposition process. Commercial single-crystalline Si substrates were
firstly cleaned in acetone for 10 min by ultrasonication. In order to remove the native
oxide layer, substrates were dipped in dilute HF solutions for 30 s and then dried
by nitrogen gas flow. The nickel chloride (NiCl2) precursor was placed in an aluminum boat at the upstream and flown by carrier gas
Ar at 30 sccm, while Si substrates were put at the downstream. The temperatures of
the precursor and substrates were controlled at 600°C and 400°C, respectively, and
held for 15 to 30 min with a 10°C/min ramping rate. The vacuum pressure was controlled
in the range of 6 to 15 Torr. The morphologies were investigated by field emission
scanning electron microscopy. XRD and TEM were utilized in structural characterization.
The noise of the atomic images was filtered by fast Fourier transform (FFT). The field
emission property was measured using a Keithley power supply (Keithly Instruments
Inc., Cleveland, OH, USA) with an anode probe of 180 μm in diameter. A superconductive
quantum interference device (SQUID; MPMS XL, SQUID Technology, Heddington, Wiltshire,
UK) was utilized for magnetic property measurements.

Results and discussion

Figure 1a,b,c,d shows the SEM images of samples grown at different pressures (6, 9, 12, 15
Torr, respectively), indicating that the geometry on the surface of substrates varied
with the ambient condition. With lower partial pressure of the precursor, as shown
in Figure 1a, Ni silicide NWs were not formed due to insufficient supply of the Ni source; however,
small nanowhiskers can be observed on the surface. As the ambient pressure was raised
to the range of 9 to 12 Torr (Figure 1b,c), NWs with high aspect ratios were obtained for proper concentrations of precursors
and growth conditions. The diameter of the NWs slightly increased with the increase
of the ambient pressure (from 30 to 50 nm to 40 to 70 nm). This may be attributed
to the fact that higher precursor concentration is more suitable for the formation
of δ-Ni2Si system. Furthermore, when the pressure was higher than 15 Torr, the concentration
of the Ni source was oversaturated and the morphology of the product turned into islands
instead of NWs. Those islands may result from the condition change to decrease the
surface energy of the system by transforming into bulk-like structures, as shown in
Figure 1d. Thus, the diameter of the NWs can be controlled under specific pressure range and
the ambient pressure plays an important role in maintaining the morphology of the
NWs.

Figure 1.SEM images of as-synthesized NWs at vacuum pressures of (a) 6, (b) 9, (c) 12, and
(d) 15 Torr. The temperature was fixed at 400°C, reaction time was 30 min, and carrier gas flow
rate was held at 30 sccm.

Figure 2a,b shows a series of SEM images of NWs with different growth times at a constant
gas flow rate (30 sccm) and ambient pressure (9 Torr). The yield and density increased
prominently when the growth time was raised from 15 to 30 min. The XRD analysis of
different reaction time is shown in Figure 2c. The characteristic peaks were examined and identified to be orthorhombic δ-Ni2Si and NiSi according to the JCPDF data base. From Figures 1 and 2, SEM images indicate that there were two types of microstructures (NWs and islands)
in the products. In order to identify each phase of the microstructures of the as-grown
products, structural analysis of the NWs has been performed. Figure 3a is the low-magnification TEM image of the NW with 30 nm in diameter. HRTEM image
(Figure 3b) shows the NW of [010] growth direction with 2-nm-thick native oxide. FFT diffraction
pattern of the lattice-resolved image is shown in the inset of Figure 3b, which represents the reciprocal lattice planes with [1] zone axis. The phase of the NW has been identified to be δ-Ni2Si, constructed with the orthorhombic structure by lattice parameters of a = 0.706 nm, b = 0.5 nm, and c =0.373 nm. Therefore, the as-deposited layer would be ascribed to NiSi.

Figure 3.Low-magnification (a) and high-resolution TEM images (b) of δ-Ni2Si NWs grown at 400°C, 9 Torr, and 30-sccm Ar flow. The image shows that there exists an oxide layer with 2 nm in thickness on the NW.
The inset in (b) shows the corresponding FFT diffraction pattern with a [1] zone axis and [010] growth
direction.

The schematic illustration of the growth mechanism is in Figure 4. In the Ni-Si binary alloy system, it has been investigated that Ni atoms are the
dominant diffusion species during the growth of orthorhombic δ-Ni2Si and NiSi [26]. The reaction and phase transformation between δ-Ni2Si and NiSi have also been reported [25]. Based on these previous studies, the reaction of the as-deposited Ni metal film
occurred to form δ-Ni2Si with a diffusion-controlled kinetics at 300°C to 400°C [27,28]. Then, partial transformation from δ-Ni2Si into NiSi thin-film structures could happen if the thickness of the Ni is below
40 nm because NiSi would form on Si substrates with a low Si/NiSi interface energy
[26,29]. Then, the continuous supply of Ni atoms may induce further growth of δ-Ni2Si phase NWs via surface diffusion kinetics [30] on the remnant δ-Ni2Si phase grains or NiSi bulks. There are two plausible and reversible formation paths
of δ-Ni2Si, which can be described in the following equations [11,24,31]:

The two equations correspond well with the experiment results: higher ambient pressure
will enhance the reaction to form Ni2Si according to LeChatelier's principle, contributing to the formation and agglomeration
of larger amount of δ-Ni2Si NWs and islands at the surface.

Due to the metallic property and special 1-D geometry, investigation of field emission
properties has been conducted. Figure 5 shows the plot of the current density (J) as a function of the applied field (E) and the inset is the ln(J/E2)−1/E plot. The sample of δ-Ni2Si NWs was measured at 10−6 Torr with a separation of 250 μm. According to the Folwer-Nordheim relationship,
the field emission behavior can be described by the following equation:

The turn-on field was defined as the applied field attained to a current density of
10 μA/cm2 and was found to be 4.12 V/μm for our Ni2Si NWs. The field enhancement factor was calculated to be about 1,132 from the slope
of the ln(J/E2)−1/E plot with the work function of 4.8 eV [32] for Ni2Si NWs. Based on the measurements, Ni2Si NWs exhibited remarkable potential applications as a field emitter like other silicide
NWs [20,25,33].

The saturated magnetization (MS) and coercivity (HC) of δ-Ni2Si NWs were measured using SQUID at 2 and 300 K, respectively. Figure 6 shows the hysteresis loop of the as-grown NWs of 30 nm in diameter with the applied
magnetic field perpendicular to the substrates. The inset highlighted the hysteresis
loop, which demonstrates a classic ferromagnetic characteristic. The HC was measured to be 490 and 240 Oe at 2 and 300 K, respectively, and MS was about 0.64 and 0.46 memu, correspondingly. For the magnetization per unit volume
(emu/cm3), normalization has been introduced through cross-sectional and plane-view SEM images
(not shown here) to estimate the density of NWs and the average volume of δ-Ni2Si NWs. The estimated values are 2.28 emu/cm3 for 2 K and 1.211 emu/cm3 for 300 K, respectively. With the normalized value, we may build up a database of
the magnetic property of Ni2Si NWs, which may be utilized in applications such as cell separation in biology [34].

Figure 6.M-H curve of δ-Ni2Si NWs measured at different temperatures. The inset is the highlight of the magnetization.

Conclusions

δ-Ni2Si phase NWs have been successfully synthesized through CVD using a single precursor,
NiCl2·6H2O. The influence of the chamber pressure on the product morphology has been discussed.
SEM, TEM, and XRD studies were conducted to analyze the growth mechanism and reaction
paths. Electrical measurements show that the field emission property of the δ-Ni2Si NWs makes them attractive choices for emitting materials. Magnetic measurements
via SQUID at different temperatures show the ferromagnetic property of the δ-Ni2Si NWs, and normalization has been applied to calculate the value of magnetization
per unit volume. This work has demonstrated future applications of Ni2Si NWs on biologic cell separation, field emitters, and magnetic storage.

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

WLC synthesized the Ni2Si nanowires. WLC and YTH performed the field emission and magnetization experiments.
JYC and CWH analyzed the diffraction data and atomic structure via TEM. CHC analyzed
the structure through XRD spectra and demonstrated the illustration of growth mechanism.
WLC and WWW conceived the study and designed the research. PHY supported the field
emission experiments. WLC, KCL, CLH, and WWW wrote the paper. All authors read and
approved the final manuscript.

Acknowledgments

WWW, CLH, and KCL acknowledge the support by National Science Council through grants
100-2628-E-009-023-MY3, 101-2218-E-008-014-MY2, and 100-2628-E-006-025-MY2.